Metal x-ray image tube



March 14, 1961 J. E. JAcoBs 2,975,313

METAL x-RAY IMAGE: TUBE Filed Jan. 8, 1959 JOHN E JACOBS ATTORNEY METAL x-RAY IMAGE TUBE John E. Jacobs, Hales Corners, Wis., assignor to General Electric Company, a corporation of New York Filed Jan. 8, 1959, Ser. N0. 785,594

2 Claims. (Cl. 313-65) The invention pertains to electric dischargetubes, and more particularly, to camera tubes having a radiation sensitive screen adapted to develop an electric signal representative of an image projected on the screen. The invention is more especially concerned with providing means in such a tube for by-passing high frequency currents that are manifested as spurious signal components superposed on the desired signal output voltage.

A typical camera tube of the storage type giving rise to the problems of stray high frequency current is one for converting an X-ray image into a video signal. Such a tube may include a hollow envelope that is enclosed at one end by an X-ray image transmitting window and at its other end by a tubular neck that houses an electron gun and electron beam deflecting plates. Adjacent the Window inside the tube there is a photoconductor target plate on which an X-ray shadowgraph or image is cast, and next to the target, but spaced and insulated therefrom, is a screen on which a suitable accelerating potential is impressed.

Ordinarily, it would seem desirable to connect the tube envelope directly to ground through a capacitor and to connect the cathode of the electron gun to ground so that these elements would be at identical potential levels and so a low impedance path for high frequency components would be inherent in the tube structure. However, experience shows that different portions of a metal envelope attain different potential levels giving rise to the equivalent of having resistance in series with a capacitor. In other words, it is only the point on the metal envelope at which the capacitor connection is made that is ordinarily truly at ground potential. The resulting resistancecapacitance circuit may act undesirably as a resonator, or even an oscillator, deriving energy from the more remote stages of the Video signal amplifying circuit or in ag-` gravated cases standing waves may even be set up on the tube.

A general purpose of the present invention is to provide an electric discharge tube that avoids the above discussed undesirable effects and which facilities effectively grounding the metal shell or tube envelope during tube operation.

Another object of this invention is the provision of a tube that has distributed capacitance about its metallic shell rather than a concentrated capacitance in its ground connection loop.

A specific object isV to provide a capacitor that is intimate with the shell of the tube in order to eliminate potential differences resulting from distributed resistances inescapably present in prior art tubes employing capacitive-by-pass circuits for high frequency current which may be generated thereindueto various tube phenomena.

Achievement ofthe foregoing and other objects will be evident when proceeding throughv the more detailed descriptionof the invention which will now be set forth in reference, to the drawing. inwhich:

Fig. 1 is a longitudinal section of an X-ray sensitive camera tube embodying the invention.

As shown in Fig. 1, the tube comprises a hollow envelope including a metal shell 10 of conicular shape which is joined at its small end 11 with a circular and` flared, cylindrical, glass neck portion 12 by conventional metal to glass sealing methods. closed at one end and fitted with a tube socket 13 according to construction and practices familiar with those practicing the art of tube manufacture.

' The wide end of the tube is enclosed by a glass window 14, in this instance. Window 14 is sutiiciently thick to withstand the atmospheric pressure differential resulting from the tube being highly evacuated, and in this example, the window glass should preferably be of a quality that transmits X-radiation with minimum absorption. At its outer margin 15, a glass to metal seal is formed between window 14 and a metal connecting ring 16 whichv latter is preferably welded to the metal cone shaped shell at a joint 17 to complete the air tight enclosure. Within the tube housing and axially adjacent window 14 there is located a target plate, generally designated by the numeralA 18. Any object intervening between window 14 and an external X-ray or other radiation source (not shown) Will, of course, cast an image or a shadowgraph on the target plate 18 inside the tube. For an X-ray image tube such as exemplified by the drawing, the target plate may comprise an aluminum substrate sheet 19 which is coated on its concave side with lead monoxide 2i) or other suitablel photoconductive material whose impedance through its thickness dimension varies substantially according to the-- intensity of radiation falling on and penetrating incremental areas of the substrate sheet 19. The thickness of the photoconductive film 20 is exaggerated in the drawing;

for the sake of clarity.

signee of the present invention.

sensitive.

Supported adjacent target 18 on insulating spacers 21 is'l a wire mesh screen 22 which in turn derives support for itself and target 18 from a ring 23 fitting tightly within@r Other details of elements for supporting the target and screen and for anchoring the: same are omitted from the drawing because they are Well' known to tube manufacturers and their discussion would .l

the conical tube envelope.

only be distracting.

In the vitreous neck 12, which in this case is glass, the tube is provided with an electron gun 30 adapted to i project a sharply focussed electron beam 31 toward thee photoconductive layer 2i) of target plate-18 when the tube '1 is electrically energized. The gun 30 may be Couven-- tional in that it contains the usual hot cathode and electrostatic focussing elements but they are not shown sincef these details are not necessary to understand the improve-- ment forming the subject of the present invention. Elec-- tron gun 30 may be supported by any suitable means in the tube neck 12 but in this case it is symbolized asl mounted on split metal rings 32 that serve to electrically` connect it with a conductive film 33 of colloidal graphite or the like deposited on the interior of the glass neck 12.

Electrostatic means are provided for defiecting the electron'beam emanating from the left end of gun 30 so as:

to causethe beam 31 to repetitively scan the entire photoconductlve surface layer 26` of the target plate on which Patented Mar. 14, 1961 The neck portionisfields when impressed with properly chosen scan voltages. A unique feature of the illustrative tube is the provision of a glass coating 36 on the interior surface of metal shell and then further coating the glass on its interior with a conductive layer 37 of graphite or vapor deposited metal, for instance. The thickness of metal shell 10, glass coating 36 and conductive layer 37 are somewhat exaggerated in the drawing. Thus, a large area capacitor is -formed where the conductive plates consisting in the metal shall 10 and internal coating 37 are separated by a dielectric layer 36 of glass or comparable material encircling the interior of the shell.

Although the dielectric layer 36- has been termed glass it is understood that this word is used in its broadest sense and is intended to include other vitreous materials. In the tube here under consideration, vitreous dielectric layer 36 is in the nature of a frit that is bonded intimatelyV with metal shell 10 by firing at relatively high temperature before the conical shell 10 is sealed onto ring 17 and neck 12.

It is desirable that dielectric layer 36 be as thin as possible in order to increase the electrostatic or capacitive intimacy between interior conductive coating 37 and cxterior metal shell 10. Satisfactory tube performance has been obtained where the glass coating; is approximately .003 inch thick, but reasonable variations from this thickness are allowable. The glass thickness, of co-urse, is dependent upon tube voltage and other parameters which those familiar with tube design will appreciate and account for. Excessive thickness of the glass layer, however, would defeat the desired intimacy between conductive layers 1t) Yand 37 and would necessitate resorting to prior art practice of covering the tube with foil and to take other measures for assuring that the exterior of the shell 'is at the same potential everywhere at its surface.

Note that the interior conductive coating 37 does not extend to the edges of the glass layer 36 but that there is an uncoated zone 38 on the glass to prevent leakage current from owing.

A conductor 39, passing through a vacuum tight insulating bushing 40 in the shell it) serves to connect target plate 18 with the irst stage of a video signal amplier, generally designated by the reference numeral 4i. Another conductor 42, passing through a similar bushing 43, connects screen 22 and interior coating 37 with an approximately 300 volt D.C. source, in this instance, such as battery 44. By reason of one side of battery 44 being grounded and the cathode of gun 30 being grounded at 45 and metal shell 10 being grounded at 4o, it is evident that screen 22 assumes full battery voltage with respect to the shell, the gun and the ground. When the tube is dark, photoconductive layer 20 accumulates electronic charge from the beam 31 projected by gun 30 until the layer reaches the potential level of the gun, whereupon no charge can reach the target plate due to the repelling influence. Likewise, when the target plate is dark, the impedance of photoconductive layer 20 is very high and only a small leakage current flows through signal resistor 47 to ground under the inuence of a positive potential from -a several hundred volt battery 48.

When X-radiation is absorbed in photoconductive layer 20, additional conductivity is produced in a pattern corresponding to the X-ray image or shadow picture. During a complete beam sweep or frame time, the surface potential on layer 20 will rise, the amount'at each point depending on conductivity. This rise will normally be a small part of a volt, and when the beam returns to a given picture element, it will recharge it to cathode potential. T'hus, the amount of charge deposited from the electron beam on a picture element will be equal to the current in that element integrated over the frame time. This recharging current also ows through signal resistor 47, since the high resistivity photoconductive material under the electron beam acts as a capacitance during theshort time it takes the electron beam to deposit charge. Thus a video signal is developed across signal resistor 47 which is in the capacitively coupled input circuit of amplifier 41.

According to the invention, any stray high frequency currents that might be developed and radiated to the tube in subsequent stages of the amplifier, not shown, or that might be impressed on the tube by interelectrode action, will be effectively by-passed in a substantially pure capacitive circuit connected directly to ground at 46 which includes the novel capacitive arrangement comprising the shell 10, glass layer 36 and conductive coating 37. A further advantage of the invention utilizing a tube envelope with a metal section is that the internal components of the tube are eiectively shielded from stray magnetic fields which in prior art tubes often aiected the electron beam landing conditions by detlecting the beam from impinging perpendicular to the target plate, thereby causing Video signal distortion.

Although the invention has been described in connection with an X-ray camera tube, such description is intended to be illustrative rather than limiting, for the invention may be embodied in various tubes employing cathode rays, and it is to be construed according to the scope'of the claims-which follow.

It is claimed:

1. An electron discharge tube comprising elements including electron gun means adapted to project a scanning electron beam, a target electrode spaced from the gun means and a screen electrode insulatingly supporting said targetv electrode transversely to the path of said beam, an evacuated envelope containing said elements, said envelope including a glass neck section surrounding said gun means and a metal section concentrically joined therewith and extending toward the target electrode, a radiation permeable window means joined with said metal section to enclose the envelope, and a vitreous dielectric coating intimately bonded with the interior surface of said metal section substantially coextensively with the length thereof, a conductive coating applied on but terminating short of said dielectric coating, and means electrically connecting said screen electrode with said conductive coating. i

2. An X-ray sensitive camera tube comprising elements including an electron gun means adapted to project a scanning electron beam, a target electrode including a photoconductive layer located remote from the gun means transversely to the path of the beam, a screen electrode interposed between the gun means and target electrode, an evacuated envelope containing the aforementioned elements, said envelope including an X-ray permeable window assembly on the gun remote side of the target electrode, a circular glass neck section surrounding said gun means, and aV conical metal shell having large and small diameter ends joined with fand interposed between the window assembly and neck section, respectively, a vitreous dielectric coating intimately bonded with the interior of said shell substantially coextensive thereof, and a conductive coating applied on but of lesser axial length than .the dielectric coating, whereby an intimate distributed capacitor is formed for by-passing stray high frequency currents around said target and screen electrodes.

' References Cited in the le of this patent y UNTTED STATES PATENTS 2,523,406

Fisch n Dec. 23, s 

